1. Field of the Invention
The present invention relates to photopolarimeters. More particularly, the present invention relates to Stokes-parameter polarimeters and spectrophotopolarimeters utilizing grating diffraction.
2. The Prior Art
Optical polarimetry is a field of vast scope and encompasses any type of optical measurement where light polarization plays an essential role, or where wave polarization itself is a carrier of information. Optical polarimetry particularly refers to the measurement of the state of polarization of light which is emitted from various sources, over a wide range of wavelengths, or which is scattered by different objects. The light sources may range from minute atomic samples to entire galaxies; and the objects may range from atoms, molecules, or microscopic samples to whole planets.
For example, ellipsometry, a branch of optical polarimetry which deals with surface and thin-film characterization by polarized-light reflection, is alone the subject of several monographs and a series of international symposia with five volumes of published proceedings. In astronomy , measurement of polarization of light, both emitted and scattered, is finding broad application for deducing microstructure of particles and macrostructure of atmospheres. Spectrophotopolarimetry has also been the main tool of mapping solar magnetic fields . Other subspecialties of optical polarimetry have witnessed similar expansion.
Many photopolarimeters have been devised to measure light polarization. For example, see P. S. Hauge, Recent Developments in Instrumentation in Ellipsometry, Surf. Sci., Vol. 96, pp. 108-140 (1980), and K. Serkowski, Polarimeters for Optical Astronomy, in Planets, Stars, and Nebulae Studied With Photopolarimetry, T Gehrels, Ed., U. of Arizona Press, Tucson 1974, pp. 135-174. A typical photopolarimeter instrument employs a single linear photodetector in front of which are placed polarizing optical elements one or more of which are subjected to some form of modulation (e.g. synchronous rotation or phase modulation). The output signal from the photodetector is Fourier analyzed to extract the four Stokes parameters of incident light as described in R. M. A. Azzam, Measurement of the Stokes parameters of light: a unified analysis of Fourier photopolarimeters, Optik, Vol. 52, pp. 253-256 (1979).
At least one photopolarimeter instrument permits the simultaneous measurement of all four Stokes parameters of light with no moving parts or modulators and requires a multichannel scheme with four (or more) detectors. This prior art photopolarimeter instrument, disclosed in R. M. A. Azzam, Arrangement of four photodetectors for measuring the state of polarization of light, Optics Lett., Vol. 10, pp. 309-311 (1985), and U.S. Pat. No. 4,681,450, issued Jul. 21, 1987, uses a spatial arrangement of four photodetectors (of which three are reflective) and no other optical components. This four-detector photopolarimeter has been implemented successfully and studied extensively and is being commercialized by Research Corporation Technologies of Tucson, Ariz.
The four-detector photopolarimeter is spectroscopic only in the sense that it can be operated over a range of wavelengths, but with light of only one wavelength incident on the four-detector photopolarimeter at a time. Therefore, the four-detector photopolarimeter is not capable of kinetic polarization analysis as a function of wavelength of incident broadband radiation. Such capability would be useful, for example, in spectroscopic ellipsometry of fast-changing physicochemical reactions on surfaces. The growing importance of this technique is underscored by the announcement of the First International Conference on Spectroscopic Ellipsometry to be held in Paris in January, 1993.
Other multichannel photopolarimeter schemes, such as those disclosed in E. Collett, Determination of the ellipsometric characteristics of optical surfaces using nanosecond laser pulses, Surf. Sci., Vol. 96, pp. 156-167 (1980), and R. Cross, B. Heffner, and P. Hernday, Polarization measurement goes automatic, Lasers and Optronics, Vol. 10, No. 11, pp. 25-26 (1991), employ the division of wavefront. Others, including R. M. A. Azzam, Division-of-amplitude photopolarimeter (DOAP) for the simultaneous measurement of all four Stokes parameters of light, Optica Acta Vol. 29, pp. 685-689 (1982) and R. M. A. Azzam, Beam-splitters for the division-of-amplitude photopolarimeter, Optica Acta, Vol. 32, pp. 1407-1412 (1985) employ the division of amplitude. In the division-of-amplitude photopolarimeter, an incident light beam, whose four Stokes parameters are to be measured, is split into four beams using an appropriately coated beam splitter and two Wollaston prisms (or equivalent birefringent or multi-layer-coated polarizing beam splitters). Linear detection of the light fluxes of the four component beams produces four output electrical signals that determine the four Stokes parameters via an instrument matrix which is obtained by calibration. The division-of-amplitude photopolarimeter is capable of fast (time-resolved) measurement of the most general state of partial elliptical polarization of light as it employs no moving parts or modulators. A variant of division-of-amplitude photopolarimeter is reported in G. E. Jellison, Four-channel polarimeter for time-resolved ellipsometry, Opt. Lett., Vol. 12, pp. 766-768 (1987), and three instruments based on the description in R. M. A. Azzam, , Division-of-amplitude photopolarimeter (DOAP) for the simultaneous measurement of all four Stokes parameters of light, Optica Acta, Vol. 29, pp. 685-689 (1982), have been operated recently as reported in K. Brudzewski, Static Stokes ellipsometer: General analysis and optimization, J. Mod. Opt., Vol. 38, pp. 889-896 (1991); S. Krishnan, Intersonics, Inc, (Northbrook Ill.) and J. Morel, Universite de Neuchatel (Switzerland).
Diffraction gratings have had a major impact on spectroscopic science and technology since their advent by Joseph Fraunhofer in 1821, who used them to measure the absorption lines of the solar spectrum. Grating diffraction has been central to spectroscopy and its many applications in virtually every branch of science and technology. An indication of their pervasive use as the dominant dispersive device in spectroscopic instruments is the claim by Bausch & Lomb (now Milton Roy) that its diffraction gratings are now used almost exclusively in hundreds of thousands of spectrophotometers and monochromators all over the world. The 1991 Physics Today Buyer's Guide lists over 30 companies that manufacture and/or sell ruled and holographic diffraction gratings. Gratings have also proved to be some of the most versatile optical science and engineering tools, finding use as waveguide couplers, wavelength division multiplexers and demultiplexers, distributed feedback elements in lasers, and holographic beam deflectors, combiners, and interconnects for optical computers.
Polarization effects that accompany grating diffraction have been known since they were first discovered by Fraunhofer. However, these effects are generally thought of as spurious and have generally not been put to practical use. A notable exception is that of the non-diffracting wire-grid polarizer which is used to linearly polarize infrared radiation (and other longer-wavelength electromagnetic waves) by the differential absorption of the component waves of incident light whose electric vectors oscillate parallel and perpendicular to the wires.
A recent paper by Todorov and Nikolova, Spectrophotopolarimeter: fast simultaneous real-time measurement of light parameters, Opt. Lett., Vol. 17, No. 5, pp. 358-359, March 1992, describes a spectrophotopolarimeter utilizing two transmission gratings in series. The first of these gratings is presumed not to change the state of polarization of light (an unrealistic assumption) and the second grating is a nonconventional one which uses the superposition of orthogonally circularly polarized waves in a photodichroic material, a grating which, to the knowledge of the present inventor, only the authors of this paper have fabricated. This reference is believed by the inventor to be the closest prior art to the present invention.